Mathematica does something stupid and incomprehensible (to me) with their series expansions. For no clear reason, it makes the O[x]n uneditable, and adds a bunch of formatting things that prevent you from copying & pasting the expression and then solving for a variable. Argh. While this is repairable using the "show expression" tool (Shift-Apple-E), it's a pain and "show expression" is not meant to be human readable.